On the magma workshops, I agree that taking it away would be very hard for most players, but we also have to agree that the huge stairway to hell wouldn't be exactly 'humanish(?)'. Maybe make it more viable with wood, outputting more charcoal for each wood used, still leading to deforestation, but still viable and not too much of a annoyance. For later game maybe instead of the magma workshop make those furnaces run on power, but maybe that's more related to the gnomes than to the humans. Perhaps the magma should be exclusive to the dwarfs, but giving the other races viable alternatives.
As for the race discussion, maybe an idea would be to include all of them on the same game, but all the specific workshops would be dependent on a first 'guild'/build that you would essentially choose which civilization you want to be part of. Special care would need to be taken so it doesn't become a lot of pages with a bunch of buildings (maybe a specific menu just for the specific workshops, divided in subgroups, making clear what which offer of different, and that the player have choices) and for once one is chosen he can't simply demolish it and choose a different one. I think this way you could still end up having a medieval armor guy with a japanese katana, but only through trade (and even that wouldn't be a necessity), without creating a total mess, mixing a bunch of different 'civs'.
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